Watermarking of digital object

ABSTRACT

A technique for identifying digital object using a digital watermark. The technique includes the steps of encrypting a message derived from source data on the digital object, to obtain an encrypted message digest (S); deriving a watermark from the encrypted message digest (S); and incorporating the watermark into the source data. The encryption is preferably done with a public key encryption system. The message to be encrypted can be obtained via performing a hash function on the source data on the digital object to obtain a message digest (M). The message digest (M) is the message encrypted with the signature encryption key to obtain the encrypted message digest (S). The watermark is resistant to cropping, scaling, and truncation.

FIELD OF THE INVENTION

[0001] The present invention is related to techniques for watermarkingdigital data, and more specifically, to watermarking digital data suchas images and audio data for authenticating copyright ownership.

BACKGROUND

[0002] Because of the rapid increase of electronic commerce in therecent years, secured data transaction is becoming more and moreimportant. To prevent electronic data to be appropriated by unauthorizedparties, cryptographic methods have been used to transmit digital databetween consenting parties to guard against unintended exposure to otherparties. Various kinds of data, including military information,financial transaction, personal data, and the like can be transmittedvia cryptography to protect the data.

[0003] In the arena of protecting the right to original art or literaryworks, in the past, most violations were by unauthorized parties makingphysical copies of authorized copies of the original (e.g., copyrighted)works. Copies of physical artistic material, such as copyrightedpaintings, photographs, phonographs, and analog audio tapes, are usuallyperceptibly inferior to the originals. The degradation of fidelity inthe copying process, e.g., in photocopying or photography, is a factorin deterring unauthorized copying of such material. Today, many visual,audio, literary, or other proprietary works are stored and transmitteddigitally. Such digital material can be copied over and over withoutsignificant loss in fidelity. The risk to the owner of an originalartistic work, or proprietary work, is that once the digital data aretransmitted, if data suspected to be copies of the transmitted data arefound, the verification of whether the suspect data are copied from theoriginally transmitted data, for example, digital data of a piece of artwork, is usually impossible.

[0004] Recently, digital watermarking has been devised as a securitytechnique to facilitate the identification of the source of digitalmaterial for the purpose of, for example, copyright enforcement. Thewatermark is an identification code that is imbedded in the originaldigital data and is preferably imperceptible to the human observer ofthe artistic work. One example of a scheme for watermarking involvesinserting an identification string into a digital audio signal tosubstitute the insignificant bits of randomly selected audio sampleswith the bits of an identification code. Another example of watermarkingrelating to watermarking video digital works involves assigning apredetermined value to a predetermined coding parameter that, whenmodified, requires a plurality of further parameters to be modified inorder to correctly decode the video signal. In one watermarkingtechnique each copy of an object is marked with an identifier code. Morerecently, a watermarking scheme in which a two-dimensional spreadspectrum signal is added to an image has been proposed. To verify thewatermark in a given image, the original image is subtracted from thegiven image and the correlation of the difference image to the watermarksignal is computed.

[0005] Although much advance has been made in watermarking digital data,generally, prior techniques of watermarking suffer from a variety ofshortcomings. Often, the original image is necessary to verify thepresence of the watermark. Manipulative operations such as cropping tocut out a portion of the work and scaling to obtain a work of larger orsmaller size pose a considerable problem on the verification process.Many times the watermarking scheme is invertible, i.e., an attacker (ortemperer), based on a first watermarked image but without knowledge ofwhat the first watermark is, would be able to compute a second image anda second watermark such that inserting the second watermark into thesecond image would result in the first watermarked image. Suchinvertible watermarking schemes can make the verification of authenticcopies of a copyrighted work difficult. Further, it may even lead toownership disputes on valuable digital data since an unscrupulous personmight appropriate another's watermarked material, subtract from it hisown watermark and claim the resulting product to be his own. Often, inprior watermarking techniques, changes in contrast or brightness mayfool the verification algorithms, making them less reliable.Furthermore, many of the watermark schemes do not offer a mechanism forcreating and managing watermarks. This means that if the same watermarkis used to protect several works, compromise in the secrecy of thewatermark will compromise the protection for all of the works. What isneeded a watermarking technique that can these deficiencies. The presentinvention provides a watermarking technique that is resistant tocropping, invertible, resistant to brightness or contrast changes, andwill not compromise other related watermarked works if the mechanism ofwatermarking in one watermarked work is disclosed.

SUMMARY

[0006] The present invention provides a technique for identifyingdigital object using a digital watermark. This technique can be easilyimplemented using computers. The technique includes encrypting dataderived from a set of source data on the digital object, deriving fromthe encrypted data a watermark, and incorporating the watermark into thesource data. Preferably, the source data of the digital object areprocessed through a hash function to obtain a message digest (M) on thedigital object and the message digest (M) is encrypted with a signatureencryption key to obtain an encrypted message digest (S). Further, theencryption of the message digest (M) is preferably done with a publickey-private key encryption system. Because the preferred mode is toprocess the source data through a one-way hash function to obtain amessage digest, for clarity and convenience, the set of data forencryption derived from the set of source data is termed a “message” or“message digest” herein although they do not necessarily have to havebeen through a one-way hash function. The set of encrypted data iscalled “encrypted message digest” herein for the same reason.

[0007] The present watermarking technique is versatile and can beadvantageously employed to watermark a variety of digital objects,including audio, video, image, multimedia data, and the like. Further,the present technique offers high security because it is not easilyfoiled by attackers. For example, by a using a hash function inconjunction with encryption in which a private key is kept confidential,the present technique offers the advantage of being not invertibility,unlike many prior watermarking techniques. Therefore, it is verydifficult for an attacker to backcompute the original watermark by thedisclosure of immediate information. To further increase security, in anembodiment of the present invention a public key encryption system isemployed. As a result, a suspect object (i.e., an object suspected to becopied from an original, e.g., copyrighted, object) can be checked withthe public key of the original object's owner to determine whether thewatermark is present without compromising the other watermarked objects.In the embodiment in which an encryption technique involving a publickey is used, ownership of an object can be established to a neutralparty, such as a court of law, using only the public key of the object'sowner, without requiring the owner to reveal his private key. Also, thewatermark is not removed by lossy compression (in which lessperceptually important information is severed to reduce the size of thedata file) or cropping (in which a portion of the object is cut). In theembodiment in which the watermarking vector is orthogonal to the vectorof the pixels into which the watermark is to be inserted, changes in thebrightness or contrast will not fool the verification algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] The following figures are included to better illustrate theembodiments of the technique of the present invention. In these figures,like numerals represent like features in the several views.

[0009]FIG. 1 is a block diagram showing an embodiment of thewatermarking technique of the present invention.

[0010]FIG. 2 is a block diagram showing in further detail an embodimentof the watermarking technique of FIG. 1

[0011]FIG. 3 shows a flow diagram of an embodiment of a watermarkedobject being obtained from a digital object according to the presentinvention.

[0012]FIG. 4 shows how a suspect object is evaluated to determinewhether it is derived from a watermarked object, according to anembodiment technique of the present invention.

[0013]FIG. 5 shows a picture printed from a digital image.

[0014]FIG. 6 shows a picture printed from a digital image having awatermark incorporated into the digital image of FIG. 5.

[0015]FIG. 7 shows a correlation spread of FIG. 6.

[0016]FIG. 8 shows a cropped and JPEG-compressed watermarked image ofFIG. 5, showing the resistance to distortion by compression andcropping.

[0017]FIG. 9 shows a correlation spread of FIG. 8, showing thesensitivity of the evaluation for the presence of watermark in theobject even when the object has been cropped and compressed.

[0018]FIG. 10 shows a truncated watermarked image of FIG. 5, showing theresistance to distortion by truncation.

[0019]FIG. 11 shows a correlation spread of FIG. 10, showing thesensitivity of the evaluation for the presence of watermark in theobject even when the object has been truncated.

DETAILED DESCRIPTION

[0020] The present invention provides a technique for inserting adigital watermark in to a digital object and for evaluating a digitalobject for the watermark. An encryption technique is used in thederivation of the watermark and the watermark is incorporated into thedigital object in such a way to render it extremely hard for an attacker(i.e., one who tampers with the watermarked data to remove or change thewatermark) to derive the original digital object from the watermarkeddigital object. The digital watermark is resistant to cropping, scaling,inadvertent distortions, as well as the intentional removal orcorruption of the watermark by an attacker.

[0021] A digital object that can be watermarked with the technique ofthe present invention is referred to as a “physical object” in that itcan be an digital visual image; digital audio program, e.g., music;digital tactile data, e.g., data which can be transformed into tactileinformation that can be sensed by touch; multimedia data; or simplydigital data strings that can be divided into discrete segmentsarrangeable into rows and columns of “pixels.” One obvious example ofsuch pixels would be, for an image, the pixels of digital informationobtained by scanning a color photograph into digital informationrepresenting rows and columns of color dots (commonly known in visualdisplay technology as “pixels”). Thus, a digitized photograph maycontain hundreds of rows and columns of pixels. However, in thisdisclosure, “pixels” can be the discrete segments of digital data forother types of digital information, e.g., those mentioned in the above.In the case of sound recording, for example, a sound signal may besampled by an A/D converter, which would output samples having valuesrepresentative of the characteristics of the sound signal at particulartime segments. The technique for obtaining pixels of data on visualimages, audio signals, and other data streams are well known in the art.Any conventional technique for obtaining the digital objects isapplicable. As another example, a digital camera or a computer running acomputer graphics software can be used to generate digital imagesdirectly. Similarly, music can be converted from sound waves to digitaldata by using A/D converters. These digital objects, as well as othertypes of digital objects, are applicable in the present invention.

[0022] An embodiment of the watermarking technique of the presentinvention is depicted in FIG. 1. A digital object, I(i,j), having m rowsand n columns of pixels, is processed through a hash function 100 (e.g.,a one-way hash function such as the MD5 function) to result in a messagedigest M. The function of the hash function is to take the input dataI(i,j) and convert them into a fixed-size string (hash), and preferablya much shorter string for a large object, much as a fingerprint, of theinput data I(i,j). Preferably, the hash is generated in such a way thatit is very difficult to generate the original input data from the hash,i.e., the message digest M. This difficulty of back calculation isbeneficial because if an attacker were able to derive the input datafrom the hash, he might be able to remove or change the watermark. Manyhash functions are known in the art and can be applied in the presenttechnique. One applicable one-way hash function (or message digestalgorithm) known in the art is the MD5 method. Other examples ofapplicable one-way hash functions include the SNEFRU function, SHAfunction, and HAVAL function. (See Schneier, B., Applied Cryptography,John Wiley and Sons, 1993, pp. 333-346 for a discussion on one-way hashfunctions.) A person skilled in the art will know how to apply such hashfunctions to a digital object. In general, in selecting a hash functionH(F) that operates on an arbitrary-length message F to obtain afixed-length hash value, h,

h=H(M),

[0023] the following characteristics are desired: Given M, it is easy tocompute h; given h, it is hard to compute M; and given M, it is hard tofind another message M′ with the property H(M)=H(M′). Preferably, thehash function is selected such that it is resistant to brute forceattacks, as well as Birthday attacks, which are based on the occurrenceof two random messages that return the same value through the same hashfunction.

[0024] The message digest M is then encrypted with a signature. Theencryption algorithm can either be a public key-private key system(asymmetric data encryption algorithm) or a private key system(symmetric data encryption algorithm). Such an encryption imparts aunique feature in the message digest M that will distinguish theseencrypted data from other data, thus acting much like a physicalsignature on a physical document. If a private key alone is used forencryption, this private key preserves the secrecy of the originalmessage and encrypted message digest is invertible, i.e., an attackercannot back calculate the message digest M or the original object fromthe encrypted message digest. However, if it is necessary to proveownership, e.g., to a third party such as a court of law, to verify theowner's signature, the owner would have to reveal his private key. Withthe private key revealed, his other watermarked objects that are markedwith the same or similar private key watermark might be compromisedsince an attacker might be able to gain access to his private key to useit to obtain information on the watermarking of the other objects.

[0025] A more preferable way would be to use a public key-private keyencryption system (referred to as the “public key system” hereinafterunless specified otherwise). In encryption using the public key system,the user possess a matched pair of keys: a private key and a public key.The private key is kept secret and is known only to the user whereas thepublic key can be distributed widely. A message encrypted with eitherkey can only be decrypted with the other key. If a user encrypts amessage with his private key, then the message can only be decryptedwith his public key. Because only the user has the private key, theencrypted message can be decrypted only with his public key. Since theprivate key is known only to the encryptor, it is established that ifthe encrypted message can be decrypted with the user's public key, hemust have encrypted the message, i.e., he has “signed” the message withhis “signature.” The strength of the signature is dependent on knowingthat the public key of the user is genuine. For this reason, public keysare preferably notarized or certified by third parties. In thisinvention, when the public key system is used, the private key is usedto encrypted the message to create the watermark from the originalimage. When it is necessary to prove ownership of a watermark, thepublic key can be provided, e.g., to a verifying third party, to verifythe owner's signature. The advantage of using public key encryption inthis invention is that since the private key is used to produce thewatermark from the original object, it will be impossible, or extremelydifficult, for an attacker to remove the watermark from the watermarkedobject to create a false original without the private key, even if hehad access to the public key.

[0026] In either the private key system case or the public key systemcase, the private key of the encryption algorithm is necessary toback-calculate from the encrypted M to obtain the original M. Manyencryption algorithms are known in the art and can be used for thispurpose. A good example is RSA algorithm. Other applicable public keyencryption algorithms include ELGAMAL algorithm and DSS (digitalsignature standard) algorithm. Many other encryption algorithms are alsoknown in the art, e.g., POHLIG-HELLMAN algorithm, RABIN algorithm, andDES (data encryption standard) algorithm, see Schneier, B., AppliedCryptography, supra. A person skilled in the art will know how to applysuch encryption algorithms to a digital object.

[0027] It is noted that the encryption step, either using a public keysystem or a private key system, can be used to encrypt the digitalobject directly without processing through a hash function, as long asthe private key is kept secret. However, if the digital image is large,extremely long computational time will be needed. The hash functionreduces the size of the data that requires encryption to produce thewatermark.

[0028] After the encrypted message digest S is formed, it is processed(block 106) to derive the watermarked object I′(i,j). In an embodiment,shown in FIG. 2, to derive the watermarked object I′(i,j), the encryptedmessage digest S is modulated, e.g., to modify the amplitude of thesignal, to spread over a perceptually significant region of the spectrum(block 108). By this process (block 108), a key vector V, representing aphysical domain signal, is obtained. Spreading the message digest S overa large part of the spectrum has the advantage of rendering thewatermark substantially imperceptible to the human sense organ for whichthe object is designed. Further, the watermark will be preserved if thedata is manipulated by processes such as compression or cropping. Forexample, spreading the message digest S over a substantial part of thespectrum would not overly distort a certain small range of colors in avisual image or distort a certain small range of audio frequency. Sincea useful watermark preferably is preserved when undergoing lossycompression or cropping, the watermark is placed in the portion of thespectrum in the perceptually significant portion of the spectralfrequencies. If such a watermarked object is cropped to remove thewatermark (e.g., by an attacker), the object would be distortedsufficiently that its quality would be substantially inferior to theoriginal object or the watermarked object. A person skilled in the artwill know what portion of the spectrum to modulate for a particularobject without undue experimentation.

[0029] From the key vector V, a watermark vector W applicable forinserting into a selected portion of the original object I(i,j) isobtained (block 110). As will be described later, the watermark vector Wis dependent on the particular portion of the original object I(i,j)selected for the insertion of the watermark. After incorporating thewatermark into the original object I(i,j) by combining the watermarkvector V with the selected portion of the original object I(i,j), thewatermarked object I′(i,j) is obtained (block 112).

[0030] To better illustrate the preferred embodiment of the invention,an example of watermarking a visual image is provided below. It is to beunderstood that other types of physical objects, for example, thosementioned above, e.g., audio digital objects, tactile digital object,and the like, can be similarly watermarked by a person skilled in theart based on the present disclosure.

EXAMPLE

[0031] The process is described in general and illustrated in FIG. 3 asfollows

[0032] (1) A digital object image I(i,j) with m rows and n column ofpixels is obtained. A message digest of the data bits in I(i,j) iscomputed, using a standard message algorithm such as the MD5 function,to obtain the message digest M.

[0033] (2) The message digest is signed to create the owner's signature,with encryption methods such as the RSA method or Elliptic Curve, thusobtaining the encrypted message digest S.

[0034] (3) Let S be the bits of the signature for constructing awatermark. For example, S may have 512 bits. A vector U of n entries isconstructed, where n corresponds to the number of columns of pixels.Alternately, the vector can have m entries corresponding to the numberof rows, m. If that is the case, the following steps referring to rowswill be applied to columns instead, and vice versa. In constructing U inthis example, the second bit to the 65th bits are assigned a modulationvalue depending on whether the corresponding bits (e.g., the first 64)of the encrypted message digest S are 0 or 1. The rest of the bits inencrypted message digest S are assigned values of 0. The first bit of Sis assigned a value of 0 to correspond to the DC component of thepixels. For example, in the second to the 65th bits of U, a bit isassigned a value of −1 for a corresponding bit of 0 in S; the bit in Uis assigned a 1 for a corresponding bit of 1 in S. It is to beunderstood that the modulation can have other optional values, as longas they are consistent and the resulting watermark would not overlydistort the object. For example, a bit in U, instead of having a valueof −1, may be assigned a value of 2; and a bit of 1 in U, may beassigned a value of −1. Furthermore, the first bits of U may have morebits or less bits than 64. However, a longer U would require morecomputer power to implement the watermark, and a smaller U is moreprompt to be broken by an attacker. The first bits of U correspond tothe lower frequencies, which are the more conceptually significantfrequencies of the visual image object. This is also true for audiodigital works. However, it is conceivable that in other works where theperceptually significant bits are in the higher frequencies, the higherfrequency bits of U will be modulated. Furthermore, the first bits of Uneed not be based on the first bits of S, but optionally can be based onsome other bits as long as they are consistent. For example, the firstbits of U can be based on the last bits, the middle bits, or alternatebits, or the like of S. The first bit in S (corresponding to the DCcomponent) having a value of 0 makes the watermark resistant to changesin brightness or contrast.

[0035] (4) A reverse Fourier transform of U is performed to obtain a keyvector V. It is to be understood that other types of transforms, e.g.,reverse Discrete Cosine Transform (DCT), can be used to transform U fromthe frequency domain to a physical domain, e.g. time domain (forexample, for audio and other time variant signals) or spatial domain(for, example, for images, video, or other spatially varying signals). Aperson skilled in the art will know how to select and apply suchtransforms to obtain the key vector V based on the present disclosure toderive a watermark.

[0036] (5) A portion of the original object I(i,j) is selected, e.g., bcontiguous rows, and this portion is averaged to construct a referencevector A relating to the image pixels for orthogonalization calculation.In this example, the reference vector A is an average vector calculatedby averaging data. Depending on the object, one may want to select thewhole object, or a portion thereof that has conceptually importantdetails. For example, b can be 16 in the middle section of the image,indicating that 16 rows in the middle section is to be averaged to formA. Then, to reduce the risk that the key vector V would have adependency on the vector A, i.e., would depend on the columnic positionof the elements in the b rows of the object, preferably the key vector Vis orthogonalized with respect to the reference vector A, thereby obtaina watermark vector W. The orthogonalization can be represented by theequation: $\begin{matrix}{{W = {{V \cdot \left( {\overset{̑}{V} \cdot Â} \right)}A}}{{{where}\quad \overset{̑}{V}} = \frac{V}{\sqrt{V \cdot V}}}{{{and}\quad Â} = \frac{A}{\sqrt{A \cdot A}}}} & {{Eq}.\quad 1}\end{matrix}$

[0037] Â is the unit vector along A. The watermark vector W representsthe frequency and magnitude data of the digital watermark that is to beincorporated into the original object I(i,j).

[0038] (6) The watermark vector W is incorporated into the originalobject I(i,j), preferably, by inserting into the portion of the I(i,j)from which the reference vector A has been derived. A common method ofinserting the watermark vector W is by adding a small scaled version ofW back to each of the b rows selected for the reference vector Aearlier. This method of obtaining the watermarked elements I′(i,j) inthe object can be represented by the following equation:

I′(i,j)=I(i,j)+a(i,j) W(i,j)  Eq. 2

[0039] In Eq. 2 a is a proportional constant, which may vary dependingon the positions of i or j if preferred.

[0040] In a preferred embodiment,

I(i,j)=c cos(2πi/b)  Eq. 3.

[0041] Typically, c is chosen such that the watermark signal is roughly−40dB PSNR (peak signal to noise ratio). Other methods of insertingwatermarks can also be used. For example, multiple scaling factors athat depend on other factors can be used. Additional watermarks can beadded to other locations of the digital object as desired, by repeatingthe orthogonalization and the watermarking steps using the same keyvector V. Alternative methods of incorporating watermark using watermarkfactors are apparent to one skilled in the art based on the presentdisclosure (see, e.g., Cox, et al. supra, which is incorporated byreference herein). In the example shown in FIG. 3, the b rows of theoriginal digital object are replaced with the b rows of watermarkedpixels. Thus the watermarked object I′(i,j) (which has the same numberof rows m and the same number of columns, n, as the original object)includes b rows of watermarked elements, whereas the remaining (m−b)rows of elements remain unchanged from the same rows in the originaldigital object. The resulting watermark is a one-dimensional watermarkin that it involves all the columns in the variation of i or j in Eqs. 2to 3, or other similar equations. This one-dimensional watermark ismathematically simpler than a two dimensional watermark in which the Wvaries as a function of the rows as well as the columns. However, ifdesired, a two-dimensional watermark can be created such that awatermark matrix W_(m) having elements that are derived from the processof one-way hash function and encryption. For example, W_(m) can bederived from V by incorporating V with various proportional constantsinto the elements at various columns and rows in W_(m). This foregoingdescription is just one example, a skilled person will know how toderive a two-dimensional watermark from the present disclosure.

[0042] Given a suspect image I″(i,j) suspected of being derived (e.g.,copied) from the watermarked object I′(i,j), the suspect image can beevaluated by the following method, which is illustrated in FIG. 4. Thismethod requires the watermark vector V and the average row vector A. Inthis method, each block of b contiguous rows in the suspect image forthe watermark is successively evaluated. For example, the first bcontiguous rows can be evaluated, then the b contiguous rows startingfrom the second row to the (1+b)th row, then the third row to the(2+b)th row, and so on.

[0043] (A) In each block to be evaluated, the b rows are averaged toobtain a reference vector A″.

[0044] (B) The vector A″ is orthogonalized with respect to A to obtain asuspect watermark vector X. The mathematics of orthogonalizing A″ issimilar to that of orthogonalizing the key vector V in Eq. 1.

[0045] (C) The relative closeness of X to W is computed, e.g., bycalculating the correlation between the W and X. The equation forcalculating the correlation between X and W a is as follows:$\begin{matrix}{{Correlation} = \frac{W \cdot X}{\sqrt{\left( {W \cdot W} \right)\left( {X \cdot X} \right)}}} & {{Eq}.\quad 4}\end{matrix}$

[0046] (D) Step (C) is repeated to compute the relative closeness of Xto W for all the blocks of contiguous b rows. The maximum of therelative closeness over all the blocks of b rows is then taken. If thisrelative closeness is above a predetermined threshold value, then thesuspect image is deemed to contain the watermark.

[0047] (E) If the suspect image I″(i,j) is cropped or scaled, ahorizontal search by searching at various match locations along thereference vector A″ can be performed to get the best relative closeness,i.e., to arrive at the maximum correlation of the watermark to thesuspect object. Further, a search can be conducted for reflections ofthe digital object about the axes.

[0048] (F) Synthesize a number of random candidate watermark vectors(e.g., one hundred watermarks) with the same spectral properties as V.Compute the correlation of each of these candidate vectors with thesuspect digital object I″(i,j), at the location, scale, and crop factorsat which the correlation of W was maximized.

[0049] (G) Compare the correlation obtained from the original watermarkagainst the correlation obtained for the random vectors. If the formerand the latter are far apart, then it is likely that the suspect objectI″(i,j) contains the watermark key vector V. In other words, it islikely that suspect object I″(i,j) has the watermark of the originalobject I(i,j), and therefore is likely to have been derived from it.

[0050] The above deals with detecting whether a suspect object isderived from an original object. In the case that the ownership of adigital object J(i,j), e.g., a digital image, is in dispute, theownership can be established via determining the presence of a watermarkby a neutral third party such as a judge. A person claiming ownership ofthe digital object will present to the judge his original image I(i,j)corresponding to J(i,j) and the signature S of the hash of that originaldigital object, and his public key for decryption. He would furtherdeclare the location in the digital object J(i,j) where the watermarkcan be found and the scaling and cropping factors of J(i,j) with respectto his original object I(i,j). The judge can verify using the followingprocess.

[0051] (A) Compute a message digest of the bits in I(i,j). Decrypt Swith the public key presented by the claimant. The bit strings for themessage digest computed and the decrypted S should be the same if theclaimant is the owner of J(i,j). Otherwise, reject the claimant's claimof ownership.

[0052] (B) Construct the watermark vector V from the signature S. At thespecified location of the object J(i,j) compute the correlation of thewatermark vector V after compensating for cropping and scaling. Use theoriginal object I(i,j) to compute the corresponding reference vector A.

[0053] (C) Synthesize random candidate watermarks with the same spectralproperties of V. Compute the correlation of each of these candidatevectors with the object J(i,j) and compare the correlation obtained withV to the correlation obtained with the random candidate vectors as inthe aforementioned method for dectecting the presence of a watermark ina suspect digital object. If the two correlation types are far apart,then it is likely the object J(i,j) contains the watermark vector V,i.e., contains the watermark of the object I(i,j) provided by theclaimant.

[0054] In this example a digital image was evaluated. The original imageobject used in this illustrative example is shown in the picture of FIG.5. The original image object has 256 (m rows)×384 (n columns) of pixels.The watermarked image is shown in FIG. 6. The strength of the watermarkis −45DBPSNR. A comparison of FIG. 5 and FIG. 6 shows that there is noperceptual difference that is distinguishable to the human eye. What canbe distinguished by the human sense organs, e.g., the eye, has beenknown in the art or can be determined without undue experimentation.FIG. 7 shows the correlation spread obtained by a verification algorithmfor the watermarked image of FIG. 6 using the above correlation method.In FIG. 7, the abscissa shows the various spurious watermarks with thesame spectral properties as the true watermark embedded in the image.The spurious watermarks were synthesized by the verification algorithm.The spike “sp” in the middle of the FIGURE corresponds to thecorrelation when applied to the image with the true watermark, whereasthe rest of the graph shows the correlation when applied to randomlygenerated watermarks and the true watermark of the same spectralbandwidth.

[0055] To evaluate the robustness of the watermark in its resistance todistortion by cropping and compression, the digital object of FIG. 5 wascropped to 176×274 pixels and JPEG_compressed with substantial loss,achieving a compression ration of 28.6 (JPEG is a standard lossycompression method, see Bhaskaran and Konstantinides, Image and Videocompression standards, Kluwer Publishers). The cropped, compressed imagewas decompressed to result in the image of FIG. 8. FIG. 9 shows thecorrelation spread for the image of FIG. 8. A tall spike “spc” isclearly seen in the middle of the spread, indicating the presence of thewatermark, thus identifying that the picture of FIG. 9 as being derivedfrom the image of FIG. 5. Similarly, making a downscaled version of theimage of FIG. 5 and computing the correlation spread of the downscaledimage (not shown in the figures) indicate that the watermarking wasverifiable after downscaling, thus showing that the watermark isresistant to scaling. Further, FIG. 10 shows the picture of the image ofFIG. 5 after truncation with setting to zero the five least significanbits of each eight-bit pixel in the watermarked image of FIG. 6. FIG. 11shows the correlation spread obtained by processing through thedetection procedure for the image of FIG. 10. Again, a clear spike “spt”is seen in the correlation spread. Thus, this evaluation on truncationshows that the watermark is resistant to truncation.

[0056] The technique of watermarking a digital object and evaluating asuspect digital object for the presence of the watermark according tothe present invention can be implemented with digital electronics thatare capable of data manipulation and calculation based on the equationsdescribed herein above. Such applicable digital electronics includemicroprocessors and computers, e.g., personal computers, minicomputers,and mainframe computers. Furthermore, the algorithms for the datamanipulation and calculation can be stored in digital storage devices,such as compact discs, floppy discs, hard discs, magnetic tapes, and thelike, which can then be loaded or read into the microprocessors orcomputers for implementation of the watermarking and evacuationprocesses. Such digital storage devices are generally articles ofmanufacture having a suitable digital storage medium readable by themicroprocessors or computers. It is also contemplated that variouscomputers can be networked so that digital objects can be transferredbetween computers to be watermarked and evaluated for watermarks. It isalso to be understood that the various steps in the watermarking processdescribed above can be done separately by different computers andprocessors and the results be combined to achieve the overall functionof watermarking or evaluation for the presence of watermark, as well asboth.

[0057] Although the preferred embodiment of the present invention hasbeen described and illustrated in detail, it is to be understood that aperson skilled in the art can make modifications within the scope of theinvention.

What is claimed is:
 1. A method for identifying digital object usingdigital watermark, comprising: (a) encrypting a message derived fromsource data on the digital object to obtain an encrypted message digest(S); and (b) deriving a watermark from the encrypted message digest (S)and incorporating into the source data.
 2. A method according to claim 1wherein the message is obtained via performing a hash function on thesource data on the digital object to obtain a message digest (M) on thedigital object, and wherein the message digest (M) is the messageencrypted with the signature encryption key to obtain the encryptedmessage digest (S).
 3. A method according to claim 1 wherein thewatermark is a physical domain watermark and the method furthercomprises incorporating the physical domain watermark to at least aportion of the source data.
 4. A method according to claim 3 furthercomprising transforming a frequency domain vector (U) derived from theencrypted message digest (S) to physical domain in deriving thewatermark.
 5. A method according to claim 4 further comprising derivingthe frequency domain vector (U) by modulating at least a portion of theencrypted message digest (S) to obtain at least a portion of the vector(U).
 6. A method according to claim 5 wherein a portion of the vector(U) corresponds to low frequencies and another portion of (U)corresponds to high frequencies, the portion of U corresponding to lowfrequencies being derived by modulating at least a portion of theencrypted message digest (S).
 7. A method according to claim 6 whereinthe portion of the vector (U) corresponding to low frequencies aremodulated to have more significant impact on amplitude of the watermarkthan the portion of the vector (U) corresponding to high frequencies. 8.A method according to claim 7 wherein the portion of the vector (U)corresponding to low frequency has negative value in elementscorresponding to “0” bits of the at least a portion of the encryptedmessage digest (S) and has positive value in elements corresponding to“1” bits of the at least a portion of the encrypted message digest (S);and wherein the portion of (U) corresponding to high frequencies haveelements of zero value.
 9. A method according to claim 1 wherein thesource data consist of rows and columns of pixels and the watermark isrepresented by a watermark vector (W) having a dimension correspondingto the number of rows (m) or the number of columns (n) of the pixels.10. A method according to claim 9 wherein a pixel contains data on adiscrete section of an image object.
 11. A method according to claim 9wherein a pixel contains data on a discrete section of audio object. 12.A method according to claim 9 wherein the watermark incorporated intothe source data is orthogonal to the data to which the watermark isadded.
 13. A method according to claim 9 further comprising derivingfrom the source data a source data vector (A) having the same dimensionas that of the watermark vector (W) by selecting at least a portion ofthe source data and further comprising deriving the watermark vector (W)based on the encrypted message digest (S) such that watermark vector (W)is orthogonal to source data vector (A); and further comprisingcombining the watermark vector (W) with the data in the selected portionof the source data from which source data vector (A) is derived to formwatermarked data.
 14. A method according to claim 9 further comprisingcomparing the at least a portion of the source data before incorporationof the watermark to after incorporation of the watermark.
 15. A methodaccording to claim 14 further comprising finding the correlation betweenthe watermark vector (W) and a target vector (X) derived from datasuspected of containing the watermark, wherein said target vector (X) isorthogonal to the source data to which the watermark is incorporated.16. A method for identifying data using digital watermark, comprising:(a) performing a one-way function on source data to obtain a messagedigest (M); (b) encrypting the message digest (M) with a signatureencryption key to obtain an encrypted message digest (S); (c) deriving afrequency domain vector (U) from the encrypted message digest (S) bymodulating a portion of the encrypted message digest (S) correspondingto low frequencies more than a portion corresponding to highfrequencies; (d) transforming the frequency domain vector (U) into aphysical domain key vector (V); (e) selecting a portion of the sourcedata and deriving a watermarking vector (W) from the frequency domainvector (U) orthogonal to the selected source data; and (f) combining theselected source data with the watermarking vector (W) in the physicaldomain.
 17. A system for identifying data using digital watermark,comprising: (a) means for encrypting a message derived from source datawith a signature encryption key to obtain an encrypted message digest(S); and (b) means for deriving a watermark from the encrypted messagedigest (S) and incorporating into the source data.
 18. A systemaccording to claim 17 further comprising a means for performing a hashfunction on the source data to obtain a message digest (M) and whereinthe means for encrypting encrypts the message digest (M) with thesignature encryption key to obtain the encrypted message digest (S). 19.A system according to claim 17 wherein the water mark is a physicaldomain watermark and the means for deriving incorporates the physicaldomain watermark to at least a portion of the source data.
 20. A systemaccording to claim 19 wherein the means for deriving derives a frequencydomain vector (U) from the encrypted message digest (S) and transformsthe vector (U) to physical domain in deriving the watermark.
 21. Asystem according to claim 20 wherein the means for deriving derives thefrequency domain vector (U) by modulating at least a portion of theencrypted message digest (S) to obtain at least a portion of the vector(U).
 22. A system according to claim 21 wherein the means for derivingmanages the source data as rows and columns of pixels and derives awatermark vector (W) based on the vector (U), the watermark vector (W)having a dimension corresponding to the number of rows (m) or the numberof columns (n) of the pixels.
 23. A system according to claim 22 whereinthe means for deriving derives from the source data a source data vector(A) having the same dimension as that of the watermark vector (W) byselecting at least a portion of the source data and wherein thewatermark vector (W) is orthogonal to source data vector (A).
 24. Asystem according to claim 23 further comprising a means for comparing aset of target data with the source data, the means for comparingcompares a target vector (X) derived from the target data to the sourcedata, the target vector (X) being orthogonal to the source data vector(A).
 25. An article of manufacture comprising a program storage medium,tangibly embodying a program code means readable by a computer to causethe computer to identifying a digital object using a digital watermark,comprising: (a) code means for performing a one-way function on sourcedata on the digital object to obtain a message digest (M) on the sourcedata; (b) code means for encrypting the message digest (M) with asignature encryption key to obtain an encrypted message digest (S); (c)code means for deriving a watermark from the encrypted message digest(S) via a transforming a portion of the message digest (S) as frequencydomain into a physical domain before resulting in a one-dimensionalwatermark for incorporating into the source data; and (d) code means forincorporating the one dimensional watermark into the source data.